You use one of the property mappers and color values of a layer to control opacity, size, or a bunch of other things relating to the particles. If you've used an effect to create your property map you must pre-compose. For example, create a new solid called ramp, add Effect>Generate>Ramp, precompose the Ramp layer moving all attributes, then select Persistent Property Mapper and choose Ramp Pre-comp as the source and map Red to Lifespan or to Opacity or Size. Where your Ramp is black you get the minimum value, where it is white you get the Maximum Value.
i don`t want to fade the particles fromm 100% to 0%.
All Particles have to live during the whole composition.
example: flock of birds which have to fly through the pictures. My birds are dying after a certain time.
I don`t like dead birds - but I havn`t an idea how to let them survive
Well, you still have to use property maps to assign properties - a simple full size solid assigned to the lifespan parameter would do. Then simply crank up the relevant parameter in the persistent property mapper. I'm wondering, though. By default, Playground's particles live forever already, so there should be no need to do anything.